Process for surface-treating body made of metal and composition of matter produced thereby

ABSTRACT

A process for surface-treating a body made of metal is disclosed, which includes a step of immersing the body made of metal in an aqueous solution containing at least one organometallic compound and at least one film-forming assistant, thereby forming a transparent film on a surface of the body made of metal. Further, in accordance with the present invention, there is provided a composition of matter including a substrate made of magnesium or a magnesium alloy and a transparent film formed on the substrate, which can be prepared by the process.

This application is a division of Ser. No. 08/802,532 filed Feb. 18,1997 now U.S. Pat. No. 5,851,597.

FIELD OF THE INVENTION

This invention relates to a process for surface-treating a body made ofmetal and a composition of matter produced by the process, and moreparticularly to a composition of matter having a transparent filmthereon and a process for surface-treating a body made of metal by whichthe composition of matter can be produced.

BACKGROUND OF THE INVENTION

Metal materials and alloy materials have been widely utilized in variousfields, for example as cladding materials, due to excellent mechanicalproperties and grave and massive appearance thereof. Especially,magnesium or magnesium alloy materials have the lightest weight amongpractically-used metal materials and exhibit a good machinability, ahigh ratio of strength to density and a good castability for die-cast,so that many studies have been made to apply the magnesium or magnesiumalloy materials to various fields such as casings, structural members orvarious parts of computers, audio equipment, communication equipment,air planes, automobiles or the like.

However, the magnesium or magnesium alloy materials is deteriorated inanti-corrosion property and therefore readily anodized in an atmosphereso that a thin oxide film is readily produced on a surface thereof.Especially, when such magnesium or magnesium alloy materials aresubjected to precision machining, there occurs a remarkable differencein an anti-corrosion property between respective surface regionsthereof. Not only the magnesium or magnesium alloy materials but alsoother general metal materials poses this problem.

Therefore, in order to enhance an anti-corrosion property, an impactresistance, a film-adhesion property and the like, the magnesium ormagnesium alloy materials have been conventionally subjected to ananodization treatment or other chemical treatments in which heavy metalsalts such as chromates (containing hexavalent (VI) chromium),manganates or permanganates, or fluorides are used.

However, when the magnesium or magnesium alloy materials undergo theanodization treatment or other chemical treatments, there arises aserious problem that an inherent metallic appearance thereof is lostthereby.

For example, in the case where the magnesium or magnesium alloymaterials are subjected to the anodization treatment or the otherchemical treatments using heavy metal salts, an effluent resulting fromeach treatment is severely contaminated by the heavy metal salts. Thisis unfavorable from the standpoint of environmental protection.

Further, when the anodization treatment is adopted, there arise thefollowing inconveniences.

That is, the anodic oxide film formed by the anodization treatment havea surface roughness three to ten times those of untreated magnesium ormagnesium alloy materials. For this reason, it is extremely difficultfor the anodized magnesium or magnesium alloy materials to attain aimeddimension after subjected to machining. Therefore, the machinedmagnesium or magnesium alloy materials are generally subjected to anabrasion process. However, since such an anodic oxide film is hard butbrittle, there is a likelihood that falling-off of the anodic oxide filmis caused at uneven sites thereof upon abrasion.

The anodic oxide film is provided with a huge number of pores eachhaving a complicated shape and a diameter on the order of 3 to 10 μm.Such powders generated during the abrasion process enters into oradhered to the pores or the uneven sites of the anodic oxide film. Whenthe powder is fallen off during the use, it functions as an abrasive sothat the anodic oxide film is apt to undergo self-destruction.

Since the anodic oxide film has a large surface roughness as describedabove, there arises a further inconvenience that the thickness thereofis difficult to control.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a processfor surface-treating a body made of metal which resolves theabove-mentioned problems.

It is another object of the present invention to provide a compositionof matter which resolves the above-mentioned problems.

According to a first aspect of the present invention, there is provideda process for surface-treating a body made of metal, which includes astep of immersing the body made of metal in an aqueous solutioncontaining at least one organometallic compound and at least onefilm-forming assistant, thereby forming a transparent film on the bodymade of metal.

According to a second aspect of the present invention, there is provideda composition of matter including a substrate and a transparent filmformed on the substrate. The substrate may be made of magnesium or amagnesium alloy. The transparent film is formed on the substrate. Thetransparent film is prepared by immersing the substrate in an aqueoussolution containing at least one organometallic compound and at leastone film-forming assistant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a characteristic curve showing the relationship between colortone of an anodic oxide film and treating time; and

FIG. 2 is a characteristic curve showing the relationship between colortone of an anodic oxide film and bath temperature.

DETAILED DESCRIPTION OF THE INVENTION

The process for surface-treating a body made of metal and thecomposition of matter produced thereby according to the presentinvention are described in detail below.

In order to achieve the above-mentioned objects, the present inventorshave conducted a variety of experiments under various conditions inwhich the utmost efforts have been made to prevent harmful substancesfrom entering in the process. As a result, it has been found that, bytreating a metal material or an alloy material with an organometalliccompound, for example, metal alkoxide, there can be obtained ahigh-quality film which is almost colorless and transparent andtherefore capable of showing a metallic appearance inherent to the metalmaterial or the alloy material as it is through the film.

The present invention has been accomplished based on the basis of thefinding. The feature of the present invention resides in that the metalmaterial or the alloy material is immersed in an aqueous solutioncontaining at least one organometallic compound and at least onefilm-forming assistant.

The metal or alloy materials to be surface-treated according to thepresent invention may include any of general kinds of metal materialsand any of general kinds of alloy materials. Especially, the presentinvention is suitably applied to magnesium or magnesium alloy materials(hereinafter referred to merely as "magnesium-based metal materials").Hitherto, the surface treatment of the magnesium-based metal materialshas been hitherto considered to be achieved only with a difficulty.

Examples of the magnesium alloy materials may include Mg/Al-basedalloys, Mg/Mn-based alloys, Mg/Ca-based alloys, Mg/Li-based alloys,Mg/Ag-based alloys, Mg/rare earth element-based alloys or the like.

In the present invention, the metal or alloy material such as theafore-mentioned magnesium-based metal material is immersed in a treatingsolution to form a film on a surface thereof. At this time, as thetreating solution, there can be used an organometalliccompound-containing aqueous solution to which a film-forming assistant(such as a film-forming accelerator or a film-forming stabilizer) isfurther added.

The suitable organometallic compounds may include, for example,metal-alkoxides, metal acetyl acetonates, metal carboxylates (salts oforganic acid and metal) or the like. At least one organometalliccompound selected from the above-enumerated compounds can be used in thepresent invention.

The metal alkoxides may be those represented by the general formula ofM(OR)_(n) where M is a metal element, R is an alkyl group and n is anoxidation number of the metal element. Examples of the metal alkoxidesusable in the present invention may include alkoxides containing asingle kind of metal, alkoxides containing two kinds of metals or thelike.

The alkoxides containing a single kind of metal may be in the form ofthe combination of a metal selected from those belonging to IA, IB, IIA,IIB, IIIA, IIIB, IVA, IVB, VB and VIB groups of the Periodic Table, andan alkyl group.

Specific examples of these alkoxides containing a single kind of metalmay include LiOCH₃, NaOCH₃, Cu(OCH₃)₂, Ca(OCH₃)₂, Sr(OC₂ H₅)₂, Ba(OC₂H₅)₂, Zn(OC₂ H₅)₂, B(OCH₃)₃, Al(iso-OC₃ H₇)₃, Ga(OC₂ H₅)₃, Y(OC₄ H₉)₃,Si(OC₂ H₅)₄, Ge(OC₂ H₅)₄, Pb(OC₄ H₉)₄, P(OCH₃)₃, Sb(OC₂ H₅)₃, VO(OC₂H₅)₃, Ta(OC₃ H₇)₅, W(OC₂ H₅)₆ or the like. In addition, the alkoxidessuch as Si(OCH₃)₄, Si(iso-OC₃ H₇)₄, Si(t-OC₄ H₉)₄, Ti(OCH₃)₄, Ti(OC₂H₅)₄, Ti(iso-OC₃ H₇)₄, Ti(OC₄ H₉)₄, Zr(OCH₃)₄, Zr(OC₂ H₅)4, Zr(OC₃ H₇)₄,Zr(OC₄ H₉)₄, Al(OC₂ H₅)₃, Al(OC₄ H₉)₃ or the like can also be used inthe present invention.

As the alkoxides containing two kinds of metals, there may be usedLa/Al-based alkoxides, Mg/Al-based alkoxides, Ni/Al-based alkoxides,Zr/Al-based alkoxides, Ba/Zr-based alkoxides or the like. Specificexamples of these alkoxides containing two kinds of metals may includeLa[Al(iso-OC₃ H₇)₄ ]₃, Mg[Al(iso-OC₃ H₇)₄ ]₃, Mg[Al(sec-OC₄ H₉)₄ ]₂,Ni[Al(iso-OC₃ H₇)₄ ]₂, (CH₃ O)₂ Zr[Al(OC₃ H₇)₄ ]₂, Ba[Zr₂ (OC₂ H₅)₉ ]₂or the like.

Example of the metal acetyl acetonates may include In(COCH₂ COOCH₃),Zn(COCH₂ COOCH₃)₂ or the like. Examples of the metal carboxylates mayinclude Pb(CH₃ COO)₂, Y(C₁₇ H₃₅ COO)₃, Ba(HCOO)₂ or the like.

The afore-mentioned organometallic compounds can be used singly or inthe form of a mixture of any two or more thereof. In any case, thecontent of the organometallic compound in the treating solution may bepreferably in the range of 0.0005 to 10 mol/liter, more preferably 0.05to 3 mol/liter. When the content of the organometallic compound in thetreating solution is less than 0.0005 mol/liter, uneven film is likelyto be produced. On the other hand, when the content of theorganometallic compound in the treating solution is more than 10mol/liter, there arises inconveniences such as the production of coloredfilm, the occurrence of a so-called "smut" or the like.

In order to accelerate or stabilize the film formation and enhance alife time of the treating solution, a film-forming assistant (so-calledfilm-forming accelerator or film-forming stabilizer) can be added to thetreating solution.

As the film-forming accelerators or film-forming stabilizers, inorganicor organic compounds may be used. Specific examples of the inorganiccompounds as the film-forming accelerators or film-forming stabilizersmay include acids such as hydrochloric acid, sulfuric acid, nitric acidor hydrofluoric acid, alkalis such as ammonia, or the like. Specificexamples of the organic compounds as the film-forming accelerators orfilm-forming stabilizers may include hydroxyl-containing compounds suchas methanol, ethanol, propanol, butanol, ethylene glycol or diethyleneglycol, carboxyl-containing compounds such as acetic acid or oxalicacid, amino-containing compounds such as triethanol amine, ethyleneoxide, xylene, formamide, dimethyl formamide, dioxyacid or the like.

These film-forming accelerators or film-forming stabilizers may be usedsingly or in the form of a mixture of any two or more thereof.

The content of the film-forming accelerator or film-forming stabilizerin the treating solution is preferably in the range of 0.0001 to 50mol/liter, more preferably 0.0005 to 10 mol/liter. When the content ofthe film-forming accelerator or film-forming stabilizer in the treatingsolution is less than 0.0001 mol/liter, the treating bath is renderedunstable. On the other hand, when the content of the film-formingaccelerator or film-forming stabilizer in the treating solution is morethan 50 mol/liter, the resulting film is apt to undergo so-called"blushing", "unevenness" or "blotches or stains", so that care must betaken upon handling and a stable anti-corrosion property thereof cannotbe obtained.

As described above, in the present invention, the metal or alloymaterial such as the magnesium-based metal material is surface-treatedby immersing in the thus-adjusted treating solution. At this time, thetemperature of a bath containing the treating solution is preferably inthe range of 0 to 50° C.

The treating time for which the metal or alloy materials aresurface-treated with the treating solution is varied, but optionallydetermined, depending upon kinds of materials to be treated, thecomposition of the treating solution, kinds of additives added to thetreating solution, the temperature of the treating bath or the like.

In accordance with the present invention, a film can be produced on asurface of the metal or alloy material without necessity of the processin which harmful substances (heavy metal salts) are used.

Accordingly, the resultant film does not contain harmful substances. Forthis reason, no environmental pollution is caused, for example, uponrecycling.

The color tone of the resultant film can be varied depending upon kindsof organometallic compounds used. For example, in the case where a metalalkoxide of Si is used as the organometallic compound, a film which iscolorless and transparent and exhibits an excellent anti-corrosionproperty can be obtained, so that a metallic appearance inherent to themetal or alloy material can be maintained.

EXAMPLES

The present invention will be described in detail hereinafter by way ofspecific experimental examples.

Experimental Example 1

This Experimental Example was conducted under various treatingconditions (including a treating time and a bath temperature).

First, a rolled plate made of magnesium (Tradename "AZ31" having a sizeof 70 mm×150 mm×31 mm) was degreased and pickled. Thereafter, the rolledplate was immersed in a treating bath maintained at 25° C. Thethus-treated rolled plate was washed with water and then dried.

The treating solution in the bath was composed of 0.67 mol/liter ofSi(OCH₃)₄ and 0.005 mol/liter of NH₄ OH.

The resultant film formed on the rolled plate was examined and evaluatedwith respect to a color tone and an anti-corrosion property thereof.

(1) Treating Time and Color Tone:

The relationship between the treating time and the color tone are shownin Table 1 below and in FIG. 1.

                  TABLE 1                                                         ______________________________________                                        Time                                                                          (min)       1         2                                                                                                        30                           ______________________________________                                        Color Color-  Color-   Color-                                                                              Color- Color-                                                                              Trans-                              tone     less     less       less                                                                                 less                                                                                less                                                                              lucent                                       and                                                                                and                      and                                             trans-                                                                           trans      trans-                                                                                trans-                                                                            trans-                                              parent                                                                           parent     parent                                                                               parent                                                                              parent                                ______________________________________                                    

As is appreciated from Table 1, in this Experimental Example, almostcolorless and transparent films were obtained even when the treatingtime was changed.

(2) Bath Temperature and Color Tone:

FIG. 2 and Table 2 show the change in color tone of the film obtainedwhen the treating time was kept unchanged but the bath temperature wasvaried.

                  TABLE 2                                                         ______________________________________                                        Bath                                                                          temperature                                                                   (° C.)                                                                            10           20                                                                                    30                                                                                           50                             ______________________________________                                        Color tone                                                                            Colorless                                                                              Colorless                                                                              Colorless                                                                            Colorless                                                                            Trans-                                                        and                                                                                   and                                                                                   and                                                                                  lucent                                              trans-                                                                                trans-                                                                                trans-                                                        parent                                                                                parent                                                                                parent                                   ______________________________________                                    

As is appreciated from Table 2 and FIG. 2, the change in color tone ofthe film was observed when the bath temperature was increased to morethan 40° C. Further, it was revealed that, when the bath temperature wasincreased to more than 50° C., the change in color tone of the filmbecame more remarkable. Accordingly, it was found that the bathtemperature was required to be adjusted to not more than 50° C. in orderto obtain a colorless and transparent film.

(3) Anti-Corrosion Property:

A magnesium plate was surface-treated in the same manner as previouslydescribed. The thus-treated magnesium plate was then subjected to a saltspray test (according to JIS Z-2371) in which an aqueous solutioncontaining 5% by weight of sodium chloride was used, and the results ofthe test were evaluated by a rating number (R. N.). Also, ordinarilyutilized anodic oxide films were tested and evaluated as ComparativeExample 1 (Dow 20) and Comparative Example 2 (Dow 21) in the same manneras above. The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                    Present    Comparative                                                                             Comparative                                  Untreated     Example      Example 1                                                                              Example 2                                 ______________________________________                                         8 hr. 10       10         10      10                                         24 hr.      9.5           10                                                                                               10                               48 hr.      9.0           9.8                                                                                              9.5                              ______________________________________                                    

As is apparent from Table 3, the test sample of the present Exampleexhibited an anti-corrosion property identical to or more excellent thanthose of the anodic oxide film samples (Comparative Examples 1 and 2).

Experimental Example 2

The procedure of Experimental Example 1 was repeated in the same manneras described above except that Si(OC₂ H₅)₄ was used instead ofSi(OCH₃)₄.

The resultant film was evaluated in the same manner as in ExperimentalExample 1. It was found that the color tone and the anti-corrosionproperty of the film of Experimental Example 2 were approximatelyidentical to those of Experimental Example 1. However, there was atendency that the change in color tone of the film occurred earlier byone rank than that of Experimental Example 1.

Experimental Example 3

The procedure of Experimental Example 1 was repeated in the same manneras described above except that a treating solution composed of 0.05mol/liter of Ti(OC₂ H₅)₄, 0.01 mol/liter of C₂ H₄ OH and 0.001 mol/literof (CH₂ COOH)₂, a bath temperature of 20° C. and a treating time of 10minutes were used.

The resultant film was slightly harder than that of Experimental Example1 and it was found that the film suffered from cracks. The color tone ofthe film was slightly white-opaque. The anti-corrosion property of thefilm was approximately identical to that of Experimental Example 1.

Experimental Example 4

The procedure of Experimental Example 1 was repeated in the same manneras described above except that a treating solution composed of 0.005mol/liter of (CH₃)Si(OC₂ H₅)₂ and 0.05 mol/liter of C₂ H₄ OH, a bathtemperature of 20° C. and a treating time of 5 minutes were used.

The resultant film exhibited slightly interference color tone but a goodanti-corrosion property.

Experimental Example 5

The procedure of Experimental Example 1 was repeated in the same manneras described above except that a treating solution composed of 0.5mol/liter of Si(OC₂ H₅)₄ and 30 mol/liter of C₂ H₄ OH, a bathtemperature of 20° C. and a treating time of 10 minutes were used.

The resultant film exhibited a good color tone and a good anti-corrosionproperty.

As described above, by using the process according to the presentinvention, it is possible to form, for example, a colorless andtransparent film having an excellent anti-corrosion property on asurface of a metal or alloy material such as magnesium-based metalmaterials, so that a metallic appearance inherent to the metal or alloymaterial can be maintained.

Further, in the process according to the present invention, since noeffluent containing heavy metals is produced, risk of causingenvironmental pollution is considerably reduced. In addition, in thecase where the surface-treated product is recycled, no specificpretreatment is required for re-melting thereof. Accordingly, theprocess for surface-treating a body of metal according to the presentinvention is favorable to environmental protection.

What is claimed is:
 1. A coated substrate consisting of:a substrate madeof magnesium or a magnesium alloy; and a transparent film formed on allsides of said substrate, said transparent film comprising at least onecompound selected from the group consisting of:(i) alkoxide compoundshaving the formula M(OR)_(n) wherein M is an element selected from thegroup consisting of Group IA, Group IB, Group IIA, Group IIB, GroupIIIA, Group IIIB, Group IVA, Group IVB, Group VB and Group VIB elementsof the periodic table, R is an alkyl group with 1 to 4 carbon atoms andn is an oxidation number of the element M; (ii) metal acetyl acetonates;and (iii) metal carboxylates; and the transparent film furthercomprising at least one film-forming assistant.
 2. A coated substrateaccording to claim 1, wherein said compound is provided in an aqueoussolution in the concentration range of 0.0005 to 10 mol/liter and saidfilm-forming assistant is provided in the aqueous solution in theconcentration range of 0.0001 to 50 mol/liter and the transparent filmis formed by immersing the substrate in the aqueous solution.
 3. Acoated substrate according to claim 2, wherein the concentration of saidcompound in the aqueous solution is in the range of 0.05 to 3 mol/literand the concentration of said film-forming assistant in the aqueoussolution is in the range of 0.0005 to 10 mol/liter.
 4. A coatedsubstrate according to claim 2, wherein said aqueous solution ismaintained at a temperature ranging from 0 to 50° C.
 5. A coatedsubstrate according to claim 1, wherein said film-forming assistant is afilm-forming accelerator or a film-forming stabilizer.
 6. A coatedsubstrate according to claim 5, wherein said film-forming accelerator orsaid film-forming stabilizer is at least one selected from the groupconsisting of acids, salts thereof, alkalis, salts thereof, andcompounds containing any of a hydroxyl group, a carboxyl group and anamino group.